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. The explanation of this is
that the blood, or fluid which takes its place, is potentially body
and flesh, or substance analogous to flesh. Now just as in
irrigation the largest dykes are permanent, while the smallest are
soon filled up with mud and disappear, again to become visible when
the deposit of mud ceases; so also do the largest blood-vessels remain
permanently open, while the smallest are converted actually into
flesh, though potentially they are no whit less vessels than before.
This too explains why, so long as the flesh of an animal is in its
integrity, blood will flow from any part of it whatsoever that is cut,
though no vessel, however small, be visible in it. Yet there can be no
blood, unless there be a blood-vessel. The vessels then are there, but
are invisible owing to their being clogged up, just as the dykes for
irrigation are invisible until they have been cleared of mud.
As the blood-vessels advance, they become gradually smaller and
smaller, until at last their tubes are too fine to admit the blood.
This fluid can therefore no longer find its way through them, though
they still give passage to the humour which we call sweat; and
especially so when the body is heated, and the mouths of the small
vessels are dilated. Instances, indeed, are not unknown of persons who
in consequence of a cachectic state have secreted sweat that resembled
blood, their body having become loose and flabby, and their blood
watery, owing to the heat in the small vessels having been too
scanty for its concoction
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